19 
connexion *ixith Physical Optics, 
sage, with its ambiguous sign prefixed in the “ unforeseen 
difficulties,^’ shows that the methods are not likely to furnish 
results accurate enough for testing important law’s of nature. 
It is not to be wondered at that the Professor, in much varia- 
tion of results, should find some which w^ere ^satisfactory ’ 
As a practical answer I here give some comparisons between 
my ow’n results and those of others. The most striking and 
most essential difference between Fresnel’s theoretical formula 
and my experimental one is, that according to the former the 
whole incident light should be reflected at 90° incidence, or 
rather, that this is the limit to which the intensity of reflection 
tends at very high incidences ; my formula gives only a quan- 
tity varying from about 72 to 76 rays reflected out of every 
100, as this limit for the ordinary kinds of crown plate and 
flint glass. Hence we have a difference which cannot be 
hidden in errors of experiment. This w’as one of Bouguer’s 
particular points. In speaking of the light reflected by water, 
he says (p. 135), “dans I’incidence pour ainsi dire infiniment 
petite la lumiere reflechie est les trois quarts de I’incidente ou 
de la directe.” We must note here that he measured his 
angle of incidence from the surface and not from the normal, 
and that a part only, instead of the whole, was reflected at the 
highest possible incidences. Below' I give Dr. Faraday’s re- 
sults with crown, plate, and flint glass, at 45° incidence (the 
only angle he used) with my own at 40° and 50°, from the 
before-named papers, as well as some calculated results. 
From Dr, Faradafs results. 
Crown glass, No. 3, reflected 
m = 4-13 
of every 100 rays. 
7, ,j 
d-3 = ^^29 
6, „ 
dl = *-32 
Plate glass, No. 2, „ 
W = 3-85 
» 
» 6, ,, 
^ = 3-88 
Flint glass. No. 9, „ 
5-13 
Trom my own results. 
Incidence. n Plate, 
reflects, of every 100. 
Flint. 
40° ... 4-94 . 
. . 4-78 
• 
5-29 
50° ... 5-68 
. . 5-92 
. . . 
6-73 
Calculations by my own formula : 
Incidence. Crown. 
Plate. 
Flint. 
40° ... 4-767 . 
. . 4-778 
... 
5-320 
45° . . . 5-205 . 
. . 5-243 
... 
5-884 
50° ... 5-810 . 
. . 5-882 
. . . 
6-656 
C2 
